[Scons-users] Thin archive Tool

Andrew C. Morrow andrew.c.morrow at gmail.com
Sat Jan 7 15:44:55 EST 2017 

Thanks Bill.

To clarify, I'm in the case of both tools, I'm only applying this override
for either static archive nodes (.a) or dynamic library nodes (.so, .dylib,
etc.). Are there by default any built-int SCons scanners that apply to such
nodes? As binary artifacts, I suspect the answer is "no".

Thanks,
Andrew


On Wed, Jan 4, 2017 at 11:57 AM, Bill Deegan <bill at baddogconsulting.com>
wrote:

> off the top of my head..
> get_content is used by scanners so overriding that could impact those.
>
> I'll have to re-digest the email thread to have smarter response..
> Probably later today.
>
> -Bill
>
>
> On Wed, Jan 4, 2017 at 6:38 AM, Andrew C. Morrow <
> andrew.c.morrow at gmail.com> wrote:
>
>>
>> Hi -
>>
>> I'm circling back to this issue for various reasons, but I'd like to
>> sidestep the questions about whether thin archives are a good idea or not,
>> or whether temp files are a solution. What I am more interested in is
>> determining the correct way to override the signature calculation for a
>> file node.
>>
>> One approach, which I'm now using in two custom Tools, is to override the
>> get_content and get_content_hash methods of FS.File. The alternative would
>> be to directly override FS.File.get_csig, and leave get_content and
>> get_content_hash as they are.
>>
>> The advantage to overriding get_content and get_content_hash is that they
>> are simple methods, and by overriding them, the existing FS.File.get_csig
>> mechanics just do the right thing. But I'm somewhat worried that there may
>> be other parts of SCons outside the signature calculation mechanism that
>> might want the actual file contents for some reason, not the "fake
>> contents" I'm returning to compute a signature for the node.
>>
>> Overriding FS.File.get_csig seems like a more targeted approach, and so
>> less likely to interfere if other parts of SCons really do need file
>> contents, but it would mean that I would need to re-create the logic of
>> FS.File.get_csig
>> <http://scons.org/doc/HTML/scons-api/SCons.Node.FS-pysrc.html#File.get_csig>
>> multiple times, and that seems likely to drift out of sync with SCons
>> itself.
>>
>> Any guidance?
>>
>> Thanks,
>> Andrew
>>
>>
>> On Mon, Oct 24, 2016 at 11:36 PM, Bill Deegan <bill at baddogconsulting.com>
>> wrote:
>>
>>> Did you try using TEMPFILE? and MAXLINELENGTH?
>>> Which I'm just seeing is far from adequately documented.
>>>
>>> see src/engine/SCons/Platform/__init__.py
>>>
>>> """A callable class.  You can set an Environment variable to this,
>>> then call it with a string argument, then it will perform temporary
>>> file substitution on it.  This is used to circumvent the long command
>>> line limitation.
>>>
>>> Example usage:
>>> env["TEMPFILE"] = TempFileMunge
>>> env["LINKCOM"] = "${TEMPFILE('$LINK $TARGET $SOURCES','$LINKCOMSTR')}"
>>>
>>> By default, the name of the temporary file used begins with a
>>> prefix of '@'.  This may be configred for other tool chains by
>>> setting '$TEMPFILEPREFIX'.
>>>
>>> env["TEMPFILEPREFIX"] = '-@'        # diab compiler
>>> env["TEMPFILEPREFIX"] = '-via'      # arm tool chain
>>> """
>>>
>>>
>>>
>>> On Mon, Oct 24, 2016 at 10:29 PM, Bill Deegan <bill at baddogconsulting.com
>>> > wrote:
>>>
>>>> Andrew,
>>>>
>>>> I guess the question is would the extra work be worth it in terms of
>>>> reduced build time?
>>>> Would covering all binutils platforms resolve the issue for you and
>>>> your team?
>>>> Or would you still be left with needing to use  thin archives on some
>>>> platforms?
>>>> Which other linkers are you dealing with?
>>>>
>>>> -Bill
>>>>
>>>> On Mon, Oct 24, 2016 at 4:47 PM, Andrew C. Morrow <
>>>> andrew.c.morrow at gmail.com> wrote:
>>>>
>>>>>
>>>>> That would probably work on binutils platforms, but I think it would
>>>>> require generating the linker script from the source list, and then feeding
>>>>> that into the link step. Without having really investigated, that
>>>>> intuitively felt like it would be more work to get SCons to do that than
>>>>> would adding some letters to ARFLAGS and synthesizing a signature for the
>>>>> target.
>>>>>
>>>>> On Mon, Oct 24, 2016 at 2:30 PM, Bill Deegan <
>>>>> bill at baddogconsulting.com> wrote:
>>>>>
>>>>>> Would something like this work?
>>>>>> https://cygwin.com/ml/cygwin/2004-04/msg00330.html
>>>>>>
>>>>>> Or is this for a non-gcc compiler?
>>>>>> -Bill
>>>>>>
>>>>>> On Mon, Oct 24, 2016 at 10:12 AM, Andrew C. Morrow <
>>>>>> andrew.c.morrow at gmail.com> wrote:
>>>>>>
>>>>>>>
>>>>>>> The short answer is that when linking by enumerating all object
>>>>>>> files on the command line, we recently hit the hard coded command line
>>>>>>> length limit on certain linux variants. By using archives to bundle groups
>>>>>>> of objects into logical libraries, we can sidestep that limitation. The
>>>>>>> downside was increased IO and disk utilization, but thin archives mitigate
>>>>>>> that.
>>>>>>>
>>>>>>> I'm still curious though - any thoughts on the implementation? Is
>>>>>>> the mechanism used for interposing on the signature calculations correct?
>>>>>>> Is there a better, more scons-y way?
>>>>>>>
>>>>>>> Thanks,
>>>>>>> Andrew
>>>>>>>
>>>>>>>
>>>>>>> On Fri, Oct 21, 2016 at 8:05 PM, Bill Deegan <
>>>>>>> bill at baddogconsulting.com> wrote:
>>>>>>>
>>>>>>>> I'm guessing that using archives let's him skip keeping track of
>>>>>>>> which object files need to be pulled in (as opposed to listing all the
>>>>>>>> object files).
>>>>>>>>
>>>>>>>> -Bill
>>>>>>>>
>>>>>>>> On Fri, Oct 21, 2016 at 11:26 AM, Rob Boehne <robb at datalogics.com>
>>>>>>>> wrote:
>>>>>>>>
>>>>>>>>> Andrew,
>>>>>>>>>
>>>>>>>>> Just out of curiosity, why are you using archives at all?  There
>>>>>>>>> are legitimate reasons, and I haven’t looked at MongoDB, but perhaps
>>>>>>>>> removing this step would simplify your build and solve this issue.
>>>>>>>>>
>>>>>>>>> HTH,
>>>>>>>>>
>>>>>>>>> Robert Boehne
>>>>>>>>>
>>>>>>>>> From: Scons-users <scons-users-bounces at scons.org> on behalf of
>>>>>>>>> "Andrew C. Morrow" <andrew.c.morrow at gmail.com>
>>>>>>>>> Reply-To: SCons users mailing list <scons-users at scons.org>
>>>>>>>>> Date: Thursday, October 20, 2016 at 3:32 PM
>>>>>>>>> To: SCons users mailing list <scons-users at scons.org>
>>>>>>>>> Subject: [Scons-users] Thin archive Tool
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Hi -
>>>>>>>>>
>>>>>>>>> The SCons based build system for MongoDB makes heavy use of static
>>>>>>>>> linking. One consequence of static linking is that the space requirements
>>>>>>>>> are basically doubled, since each translation unit produces an object file,
>>>>>>>>> and then each object file is copied into an archive file. Adding CacheDir
>>>>>>>>> into the mix multiplies this duplication.
>>>>>>>>>
>>>>>>>>> The GNU binutils tools, however, support 'thin' archives, where
>>>>>>>>> the archive contents are simply a list of file references, meaning that the
>>>>>>>>> archive files are very small. At link time, the linker simply dereferences
>>>>>>>>> the listed files in each archive.
>>>>>>>>>
>>>>>>>>> To support this, we added the following Apache 2.0 licensed tool:
>>>>>>>>>
>>>>>>>>> https://github.com/mongodb/mongo/blob/master/site_scons/site
>>>>>>>>> _tools/thin_archive.py
>>>>>>>>>
>>>>>>>>> One subtle aspect to consider is that when using thin archives, if
>>>>>>>>> application X depends on libY.a, which contains z.o produced from z.c, then
>>>>>>>>> if z.c is changed, the built-in signature of libY.a will not change, since
>>>>>>>>> the reference to z.o doesn't change when the archive is rebuilt, so taking
>>>>>>>>> the MD5 of the file contents will yield the same result.
>>>>>>>>>
>>>>>>>>> To address this, our Tool creates a new Node subclass that
>>>>>>>>> overrides the get_contents and get_content_hash methods, and sets the
>>>>>>>>> target_factory for StaticLibrary to produce that Node subclass. The
>>>>>>>>> overriding behavior computes a content hash based on the content hash of
>>>>>>>>> the children of the new node.
>>>>>>>>>
>>>>>>>>> This all seems to work, fairly well, but I was curious if there
>>>>>>>>> was a more appropriate way to accomplish this. The end goal is that we want
>>>>>>>>> the content signature of these Nodes to be a hash of the signatures of all
>>>>>>>>> of the Nodes children, rather than the on-disk contents of the Node.
>>>>>>>>>
>>>>>>>>> Is there a better way to accomplish this than what we are doing
>>>>>>>>> here?
>>>>>>>>>
>>>>>>>>> FWIW we also use a similar technique for driving ABI change
>>>>>>>>> linking when doing dynamic builds:
>>>>>>>>>
>>>>>>>>> https://github.com/mongodb/mongo/blob/master/site_scons/site
>>>>>>>>> _tools/abilink.py
>>>>>>>>>
>>>>>>>>> Here, we would be particularly interested in arranging to
>>>>>>>>> interpose in such a way to absolutely minimize the number of times we must
>>>>>>>>> invoke abidw, as that is very expensive. Avoiding needless re-invocations
>>>>>>>>> of abidw as things move in and out of the CacheDir is desired.
>>>>>>>>>
>>>>>>>>> Thanks,
>>>>>>>>> Andrew
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> _______________________________________________
>>>>>>>>> Scons-users mailing list
>>>>>>>>> Scons-users at scons.org
>>>>>>>>> https://pairlist4.pair.net/mailman/listinfo/scons-users
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
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>>>>>>>>
>>>>>>>
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>>>>>>>
>>>>>>
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>>>>>>
>>>>>
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>>>>
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